This invention relates generally to pressure control valves and, more particularly, to a pressure control valve operated by a solenoid.
Pressure control valves have long been used to provide a precise pressure in hydraulic applications where maintenance of selectively variable fluid pressures is required. One such application is a hydraulic railway braking system for transit cars, such as are used by the transit cars of the Washington Metropolitan Transit Authority (WMATA). Such a system is illustrated in U.S. Pat. No. Re 29,096--Jones, which is incorporated herein by reference. In the system disclosed in the Jones patent, a brake control valve 5-21 is a pilot-controlled pressure control valve which is operable to provide a variable hydraulic pressure to the brakes 6-0 to provide varying levels of braking, or brake rates. Such systems can be either hydraulic-apply, spring-release, or spring-apply, hydraulic-release systems.
The pressure control valves used on WMATA transit cars are typical of most pressure control valves in general in that it is pilot-controlled. In such a pilot-controlled valve, brake or outlet pressure acting on the end of the valve spool is opposed by a hydraulic pilot pressure which is selectively variable to control outlet pressure, which controls braking level or force. Such valves have long been used in a variety of industrial applications. However, such valves require a constant pilot fluid flow to maintain proper pilot pressure, which is normally non-problematical in an industrial application.
However, in a transit application, this constant pilot flow can be problematical. In WMATA transit cars, a drop in the pressure of the pilot fluid results in a fail-safe automatic application of the car's emergency brakes. This pilot pressure is supplied from an accumulator which also supplies the main brake pressure fluid. The accumulator is continually drained to provide pressure fluid for valve and brake operation and must be periodically replenished by operation of an electric pump. This pump is powered by electric current derived from the transit system's "third rail", which provides electric power through a pickup shoe to the transit car for propulsion, lighting, braking and other functions.
Since a plurality of power stations provide third rail power to a plurality of isolated power zones in the transit system, the third rail is not continuous. If a transit car is stopped where a car's third rail pickup shoe is in one of these third rail gaps, all power is cut off to the car, except power from the car's battery. If the interval of car stoppage in the third rail gap is sufficiently long, the accumulator supplying pilot pressure fluid to the brake control valve will drain until insufficient pressure is available and the car's brakes will automatically go into emergency braking.
Thus the car and the train will be immovable until the brakes on the affected car are "cut out". This removes all braking on that car and reduces the total braking available to the train. A train so crippled cannot be used in regular revenue service until power can be restored. This is usually at least a fifteen minute procedure, during which all transit operations in that portion of the system are delayed. Thereafter, the braking on the affected car will have to be manually "cut in" to render the train's braking system whole again. Such a situation is a major inconvenience to operation of the transit system in terms of both time and expense.
As stated above, use of a pilot-controlled brake control valve requires a constant flow of pilot fluid to maintain the predetermined pilot pressure. The pilot flow requirements of this pilot flow brake control valve and the limited size of the accumulator used on WMATA transit cars require the use of a pump driven by a 1 horsepower motor that cycles every 3 minutes in normal usage to replenish the accumulator. This rapid cycling of the pump causes such wear and tear on the pumps as to necessitate frequent replacement of the pumps. Rapid pump cycling consumes substantial electric energy and is very energy inefficient.
A further inconvenience of the current pilot-controlled brake control valve in use on WMATA transit cars occurs during movement of a car into the maintenance shops. During such movement, the cars move off third rail power into a propulsion "coast" mode. If this occurs just before the accumulator calls for recharge, no recharge is possible thereafter and a call for braking can dump the car into emergency braking mode. It is then necessary to provide power to the car from a mobile source, causing delay and inconvenience.
It would be desirable to provide a pressure control valve which is energy efficient.
It would also be desirable to provide a pressure control valve which drastically reduces or eliminates the need for a constant pilot fluid flow.
It would be further desirable to provide a pressure control valve for use as a transit brake control valve which so drastically reduces the accumulator drainage that the frequency of replenishment cycle intervals is drastically reduced and a much smaller replenishment electric motor can be used.